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RFC 8640 - Dynamic Subscription to YANG Events and Datastores ov

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Internet Engineering Task Force (IETF)                           E. Voit
Request for Comments: 8640                                 Cisco Systems
Category: Standards Track                                       A. Clemm
ISSN: 2070-1721                                                Futurewei
                                                      A. Gonzalez Prieto
                                                       E. Nilsen-Nygaard
                                                             A. Tripathy
                                                           Cisco Systems
                                                          September 2019

    Dynamic Subscription to YANG Events and Datastores over NETCONF


   This document provides a Network Configuration Protocol (NETCONF)
   binding to the dynamic subscription capability of both subscribed
   notifications and YANG-Push.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Compatibility with <create-subscription> as Defined in
      RFC 5277 ........................................................4
   4. Mandatory XML, Event Stream, and Datastore Support ..............4
   5. NETCONF Connectivity and Dynamic Subscriptions ..................4
   6. Notification Messages ...........................................5
   7. Dynamic Subscriptions and RPC Error Responses ...................5
   8. Security Considerations .........................................7
   9. IANA Considerations .............................................7
   10. References .....................................................7
      10.1. Normative References ......................................7
      10.2. Informative References ....................................8
   Appendix A. Examples ...............................................9
     A.1. Event Stream Discovery ......................................9
     A.2. Dynamic Subscriptions ......................................10
     A.3. Subscription State Notifications ...........................15
     A.4. Filter Examples ............................................17
   Acknowledgments ...................................................19
   Authors' Addresses ................................................19

1.  Introduction

   This document specifies the binding of a stream of events that form
   part of a dynamic subscription to the Network Configuration Protocol
   (NETCONF) [RFC6241].  Dynamic subscriptions are defined in [RFC8639].
   In addition, as [RFC8641] is itself built upon [RFC8639], this
   document enables a NETCONF client to request via a dynamic
   subscription, and receive, updates from a YANG datastore located on a
   NETCONF server.

   This document assumes that the reader is familiar with the
   terminology and concepts defined in [RFC8639].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are defined in [RFC8639]: dynamic subscription,
   event stream, notification message, publisher, receiver, subscriber,
   and subscription.  This document does not define any additional

3.  Compatibility with <create-subscription> as Defined in RFC 5277

   A publisher is allowed to concurrently support dynamic subscription
   RPCs as defined in [RFC8639] at the same time as the
   <create-subscription> RPC defined in [RFC5277].  However, a single
   NETCONF transport session MUST NOT support both this specification
   and a subscription established by the <create-subscription> RPC
   defined in [RFC5277].  To protect against any attempts to use a
   single NETCONF transport session in this way:

   o  A solution MUST reply with the <rpc-error> element [RFC6241]
      containing the "error-tag" value of "operation-not-supported" if a
      <create-subscription> RPC is received on a NETCONF session where
      an established subscription per [RFC8639] exists.

   o  A solution MUST reply with the <rpc-error> element [RFC6241]
      containing the "error-tag" value of "operation-not-supported" if
      an "establish-subscription" request has been received on a NETCONF
      session where the <create-subscription> RPC [RFC5277] has
      successfully created a subscription.

   If a publisher supports this specification but not subscriptions via
   [RFC5277], the publisher MUST NOT advertise

4.  Mandatory XML, Event Stream, and Datastore Support

   The "encode-xml" feature of [RFC8639] MUST be supported.  This
   indicates that XML is a valid encoding for RPCs, state change
   notifications, and subscribed content.

   A NETCONF publisher supporting event stream subscription via
   [RFC8639] MUST support the "NETCONF" event stream identified in that

5.  NETCONF Connectivity and Dynamic Subscriptions

   Management of dynamic subscriptions occurs via RPCs as defined in
   [RFC8641] and [RFC8639].  For a dynamic subscription, if the NETCONF
   session involved with the "establish-subscription" terminates, the
   subscription MUST be terminated.

   For a dynamic subscription, any "modify-subscription",
   "delete-subscription", or "resync-subscription" RPCs MUST be sent
   using the same NETCONF session upon which the referenced subscription
   was established.

6.  Notification Messages

   Notification messages transported over NETCONF MUST be encoded in a
   <notification> message as defined in [RFC5277], Section 4.  And per
   the <eventTime> object definition provided in [RFC5277], <eventTime>
   is populated with the event occurrence time.

   For dynamic subscriptions, all notification messages MUST use the
   NETCONF transport session used by the "establish-subscription" RPC.

7.  Dynamic Subscriptions and RPC Error Responses

   When an RPC error occurs as defined in [RFC8639], Section 2.4.6 and
   [RFC8641], Appendix A, the NETCONF RPC reply MUST include an
   <rpc-error> element per [RFC6241] with the error information
   populated as follows:

   o  An "error-type" node of "application".

   o  An "error-tag" node, where the value is a string that corresponds
      to an identity associated with the error.  For the mechanisms
      specified in this document, this "error-tag" will correspond to
      the error identities in either (1) [RFC8639], Section 2.4.6, for
      general subscription errors:

         error identity          uses error-tag
         ----------------------  -----------------------
         dscp-unavailable        invalid-value
         encoding-unsupported    invalid-value
         filter-unsupported      invalid-value
         insufficient-resources  resource-denied
         no-such-subscription    invalid-value
         replay-unsupported      operation-not-supported

      or (2) [RFC8641], Appendix A.1, for subscription errors specific
      to YANG datastores:

         error identity               uses error-tag
         ---------------------------  -----------------------
         cant-exclude                 operation-not-supported
         datastore-not-subscribable   invalid-value
         no-such-subscription-resync  invalid-value
         on-change-unsupported        operation-not-supported
         on-change-sync-unsupported   operation-not-supported
         period-unsupported           invalid-value
         update-too-big               too-big
         sync-too-big                 too-big
         unchanging-selection         operation-failed

   o  An "error-severity" of "error" (this MAY be included).

   o  An "error-app-tag" node, where the value is a string that
      corresponds to an identity associated with the error, as defined
      in [RFC8639], Section 2.4.6 for general subscriptions and
      [RFC8641], Appendix A.1 for datastore subscriptions.  The specific
      identity to use depends on the RPC for which the error occurred.
      Each error identity will be inserted as the "error-app-tag"
      following the form <modulename>:<identityname>.  An example of
      such a valid encoding would be
      "ietf-subscribed-notifications:no-such-subscription".  Viable
      errors for different RPCs are as follows:

         RPC                     has base identity
         ----------------------  ----------------------------
         establish-subscription  establish-subscription-error
         modify-subscription     modify-subscription-error
         delete-subscription     delete-subscription-error
         kill-subscription       delete-subscription-error
         resync-subscription     resync-subscription-error

   o  In the case of error responses to an "establish-subscription" or
      "modify-subscription" request, there is the option of including an
      "error-info" node.  This node may contain XML-encoded data with
      hints for parameter settings that might lead to successful RPC
      requests in the future.  The yang-data structures from [RFC8639]
      and [RFC8641] that may be returned are as follows:

      establish-subscription returns hints in yang-data structure
      ---------------------- -------------------------------------------
      target: event stream   establish-subscription-stream-error-info
      target: datastore      establish-subscription-datastore-error-info

      modify-subscription    returns hints in yang-data structure
      ---------------------- ----------------------------------------
      target: event stream   modify-subscription-stream-error-info
      target: datastore      modify-subscription-datastore-error-info

      The yang-data included in "error-info" SHOULD NOT include the
      optional leaf "reason", as such a leaf would be redundant with
      information that is already placed in the "error-app-tag".

   In the case of an RPC error resulting from a "delete-subscription",
   "kill-subscription", or "resync-subscription" request, no
   "error-info" needs to be included, as the "subscription-id" is the
   only RPC input parameter and no hints regarding this RPC input
   parameter need to be provided.

8.  Security Considerations

   This document does not introduce additional security considerations
   for dynamic subscriptions beyond those discussed in [RFC8639].  But
   there is one consideration worthy of more refinement based on the
   connection-oriented nature of NETCONF.  Specifically, if a buggy or
   compromised NETCONF subscriber sends a number of "establish-
   subscription" requests, then these subscriptions accumulate and may
   use up system resources.  In such a situation, subscriptions MAY be
   terminated by terminating the underlying NETCONF session.  The
   publisher MAY also suspend or terminate a subset of the active
   subscriptions on that NETCONF session in order to reclaim resources
   and preserve normal operation for the other subscriptions.

9.  IANA Considerations

   This document has no IANA actions.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
              Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,

   [RFC8639]  Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
              E., and A. Tripathy, "Subscription to YANG Notifications",
              RFC 8639, DOI 10.17487/RFC8639, September 2019,

   [RFC8641]  Clemm, A. and E. Voit, "Subscription to YANG Notifications
              for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
              September 2019, <https://www.rfc-editor.org/info/rfc8641>.

              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation
              REC-xml-20081126, November 2008,

10.2.  Informative References

   [RFC8347]  Liu, X., Ed., Kyparlis, A., Parikh, R., Lindem, A., and M.
              Zhang, "A YANG Data Model for the Virtual Router
              Redundancy Protocol (VRRP)", RFC 8347,
              DOI 10.17487/RFC8347, March 2018,

   [XPATH]    Clark, J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", November 1999,

Appendix A.  Examples

   This appendix is non-normative.  Additionally, the subscription "id"
   values of 22, 23, 39, and 99 used below are just examples.  In
   production, the actual values of "id" might not be small integers.

A.1.  Event Stream Discovery

   As defined in [RFC8639], an event stream exposes a continuous set of
   events available for subscription.  A NETCONF client can retrieve the
   list of available event streams from a NETCONF publisher using the
   <get> operation against the top-level "streams" container defined in
   [RFC8639], Section 3.1.

   The following XML example [W3C.REC-xml-20081126] illustrates the
   retrieval of the list of available event streams:

<rpc message-id="101"
    <filter type="subtree">

          Figure 1: <get> Request for Retrieval of Event Streams

   After such a request, the NETCONF publisher returns a list of
   available event streams as well as additional information that might
   exist in the container.

A.2.  Dynamic Subscriptions

A.2.1.  Establishing Dynamic Subscriptions

   Figure 2 shows two successful "establish-subscription" RPC requests
   as per [RFC8639].  The first request is given a subscription "id"
   of 22, and the second is given an "id" of 23.

              +------------+                 +-----------+
              | Subscriber |                 | Publisher |
              +------------+                 +-----------+
                    |                              |
                    |    Capability Exchange       |
                    |                              |
                    |                              |
                    |    establish-subscription    |
                    |----------------------------->|  (a)
                    | RPC Reply: OK, id = 22       |
                    |<-----------------------------|  (b)
                    |                              |
                    | notification message (for 22)|
                    |                              |
                    |                              |
                    |    establish-subscription    |
                    | notification message (for 22)|
                    | RPC Reply: OK, id = 23       |
                    |                              |
                    |                              |
                    | notification message (for 22)|
                    | notification message (for 23)|
                    |                              |

          Figure 2: Multiple Subscriptions over a NETCONF Session

   To provide examples of the information being transported, example
   messages for interactions (a) and (b) in Figure 2 are detailed below
   (Figures 3 and 4):

<rpc message-id="102" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
    <stream-xpath-filter xmlns:ex="https://example.com/events">

              Figure 3: "establish-subscription" Request (a)

   As the NETCONF publisher was able to fully satisfy the request (a),
   the publisher sends the subscription "id" of the accepted
   subscription in its reply message (b):

  <rpc-reply message-id="102"

            Figure 4: A Successful "establish-subscription" (b)

   If the NETCONF publisher had not been able to fully satisfy the
   request or the subscriber has no authorization to establish the
   subscription, the publisher would have sent an RPC error response.
   For instance, if the "dscp" value of 10 asserted by the subscriber in
   Figure 3 proved unacceptable, the publisher may have returned:

   <rpc-reply message-id="102"

            Figure 5: An Unsuccessful "establish-subscription"

   The subscriber can use this information in future attempts to
   establish a subscription.

A.2.2.  Modifying Dynamic Subscriptions

   An existing subscription may be modified.  The following exchange
   shows a negotiation of such a modification via several exchanges
   between a subscriber and a publisher.  This negotiation consists of a
   failed RPC modification request/response followed by a
   successful one.

              +------------+                 +-----------+
              | Subscriber |                 | Publisher |
              +------------+                 +-----------+
                    |                              |
                    | notification message (for 23)|
                    |                              |
                    | modify-subscription (id = 23)|
                    |----------------------------->|  (c)
                    | RPC error (with hint)        |
                    |<-----------------------------|  (d)
                    |                              |
                    | modify-subscription (id = 23)|
                    | RPC Reply: OK                |
                    |                              |
                    | notification message (for 23)|
                    |                              |

   Figure 6: Interaction Model for Successful Subscription Modification

   If the subscription being modified in Figure 6 is a datastore
   subscription as per [RFC8641], the modification request made in (c)
   may look like that shown in Figure 7.  As can be seen, the
   modifications being attempted are the application of a new XPath
   filter as well as the setting of a new periodic time interval.

<rpc message-id="303"
    <yp:datastore-xpath-filter xmlns:ex="https://example.com/datastore">

              Figure 7: Subscription Modification Request (c)

   If the NETCONF publisher can satisfy both changes, the publisher
   sends a positive result for the RPC.  If the NETCONF publisher cannot
   satisfy either of the proposed changes, the publisher sends an RPC
   error response (d).  Figure 8 shows an example RPC error response for
   (d) that includes a hint.  This hint is an alternative time period
   value that might have resulted in a successful modification:

   <rpc-reply message-id="303"

           Figure 8: "modify-subscription" Failure with Hint (d)

A.2.3.  Deleting Dynamic Subscriptions

   Figure 9 demonstrates the deletion of a subscription.  This
   subscription may have been to either a stream or a datastore.

  <rpc message-id="103"

                      Figure 9: "delete-subscription"

   If the NETCONF publisher can satisfy the request, the publisher
   returns a reply indicating success.

   If the NETCONF publisher cannot satisfy the request, the publisher
   sends an <rpc-error> element indicating that the modification didn't
   work.  Figure 10 shows a valid response for an existing valid
   subscription "id", but that subscription "id" was created on a
   different NETCONF transport session:

   <rpc-reply message-id="103"

             Figure 10: An Unsuccessful "delete-subscription"

A.3.  Subscription State Notifications

   A publisher will send subscription state notifications for dynamic
   subscriptions according to the definitions in [RFC8639].

A.3.1.  "subscription-modified"

   As per Section 2.7.2 of [RFC8639], a "subscription-modified" might be
   sent over NETCONF if the definition of a configured filter changes.
   A subscription state notification encoded in XML would look like:

<notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
    <stream-xpath-filter xmlns:ex="https://example.com/events">

    Figure 11: "subscription-modified" Subscription State Notification

A.3.2.  "subscription-resumed" and "replay-complete"

   A "subscription-resumed" would look like:


              Figure 12: "subscription-resumed" Notification

   The "replay-complete" is virtually identical, with "subscription-
   resumed" simply being replaced by "replay-complete".

A.3.3.  "subscription-terminated" and "subscription-suspended"

   A "subscription-terminated" would look like:


   Figure 13: "subscription-terminated" Subscription State Notification

   The "subscription-suspended" is virtually identical, with
   "subscription-terminated" simply being replaced by "subscription-

A.4.  Filter Examples

   This appendix provides examples that illustrate both XPath and
   subtree methods of filtering event record contents.  The examples are
   based on the YANG notification "vrrp-protocol-error-event" as defined
   per the ietf-vrrp YANG data model in [RFC8347].  Event records based
   on this specification that are generated by the publisher might
   appear as:

  <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">

             Figure 14: Example VRRP Notification per RFC 8347

   Suppose that a subscriber wanted to establish a subscription that
   only passes instances of event records where there is a
   "checksum-error" as part of a Virtual Router Redundancy Protocol
   (VRRP) protocol event.  Also, assume that the publisher places such
   event records into the NETCONF stream.  To get a continuous series of
   matching event records, the subscriber might request the application
   of an XPath filter against the NETCONF stream.  An "establish-
   subscription" RPC to meet this objective might be:

 <rpc message-id="601" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <stream-xpath-filter xmlns="urn:ietf:params:xml:ns:yang:ietf-vrrp">

       Figure 15: Establishing a Subscription Error Reason via XPath

   For more examples of XPath filters, see [XPATH].

   Suppose that the "establish-subscription" in Figure 15 was accepted.
   And suppose that a subscriber decided later on that they wanted to
   broaden this subscription to cover all VRRP protocol events (i.e.,
   not just those with a "checksum-error").  The subscriber might
   attempt to modify the subscription in a way that replaces the XPath
   filter with a subtree filter that sends all VRRP protocol events to a
   subscriber.  Such a "modify-subscription" RPC might look like:

 <rpc message-id="602" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">

               Figure 16: Example "modify-subscription" RPC

   For more examples of subtree filters, see [RFC6241], Section 6.4.


   We wish to acknowledge the helpful contributions, comments, and
   suggestions that were received from Andy Bierman, Yan Gang, Sharon
   Chisholm, Hector Trevino, Peipei Guo, Susan Hares, Tim Jenkins,
   Balazs Lengyel, Martin Bjorklund, Mahesh Jethanandani, Kent Watsen,
   Qin Wu, and Guangying Zheng.

Authors' Addresses

   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com

   Alexander Clemm

   Email: ludwig@clemm.org

   Alberto Gonzalez Prieto

   Email: alberto.gonzalez@microsoft.com

   Einar Nilsen-Nygaard
   Cisco Systems

   Email: einarnn@cisco.com

   Ambika Prasad Tripathy
   Cisco Systems

   Email: ambtripa@cisco.com


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